Arteriosclerosis is one of the lifestyle-related diseases in which excessive lifestyle habits are risk factors. Arteriosclerosis advances from youth with very few symptoms, and if left untreated, arteriosclerosis may develop into cerebral infarction, cardiac infarction, angina pectoris, and the like. It is difficult to cure arteriosclerosis. Therefore, it is important to find symptoms of arteriosclerosis at an early stage and prompt the patients to improve lifestyle habits to slow down the progression of the diseases.
There are various diagnostic methods for examining arteriosclerosis. Among the methods, ultrasonic echo examination allows noninvasive direct observation of blood vessels and is an indispensable examination in the current clinical diagnosis. The carotid artery ultrasonic echo examination for arteriosclerosis diagnosis evaluates the presence of plaque and intima-media complex thickness (IMT). However, at an early stage of arteriosclerosis, there is no plaque, and the IMT is thin. It is difficult to find symptoms at an early stage of arteriosclerosis in the morphological observation by ultrasonic echo examination.
As contrasted with this, since arteriosclerosis is sclerosis of artery and loss of elasticity, there is an idea that movements of artery associated with heartbeats can be evaluated. Various indicators of the degree of arteriosclerosis based on evaluations related to the excellence of behaviors of artery have been proposed.
For example, Non-Patent Documents 1 and 2 propose stiffness parameters and modified stiffness parameters, respectively.
                              β          *                =                                            ln              ⁢                                                          ⁢                              P                s                                      -                          ln              ⁢                                                          ⁢                              P                d                                                          (                                                            D                  s                                -                                  D                  d                                                            D                d                                      )                                              (        1        )            
The above Equation (1) shows a calculation of the modified stiffness parameter. In Equation (1), Ps denotes blood pressure systolic of a carotid artery, Pd denotes blood pressure diastolic of the carotid artery, Ds denotes maximum diameter of the carotid artery at blood pressure systolic, and Dd denotes minimum diameter of the carotid artery at blood pressure diastolic. In Equation (1), the difference between the blood pressure systolic and blood pressure diastolic of the carotid artery is normalized by the excellence of behavior of the carotid artery. The Equation (1) indicates that the greater the value of β* is, the worse the behavior of carotid artery becomes considering the blood pressure difference, or the stiffer the carotid artery is.
In Patent Document 1, the carotid artery is assumed to be a thick cylinder with a thickness d, and an elastic modulus Eth of the carotid artery is calculated according to the following Equation (2) based on the mechanical characteristics of distortion and stress. In Equation (2), d denotes the thickness of a blood vessel wall.
                              E          th                =                                            P              s                        -                                          P                d                            ⁢                                                D                  d                                ⁡                                  (                                                            D                      d                                        +                                          2                      ⁢                      d                                                        )                                                                                                                          D                  s                                -                                  D                  d                                                            D                d                                      ⁢            2            ⁢                          d              ⁡                              (                                                      D                    d                                    +                  d                                )                                                                        (        2        )            
Surrounding tissues, such as fat, existing around the carotid artery are not taken into consideration in all the documents described above. However, the behavior of a carotid artery is influenced by the surrounding tissues in no small part. The conventional methods cannot distinguish whether the restricted movement of a carotid artery is caused by sclerosis of the carotid artery or by stiffness of the surrounding tissues of the carotid artery.
Patent Document 1: Japanese Patent No. 3882084
Non-Patent Document 1: Hayashi K, Nakagawa S, Naruto Y, Moritake K, Okumura A, “Parametric description of mechanical behavior of arterial walls”, Journal of Japanese Society of Biorheology, 3 (1980), pp. 75-78.
Non-Patent Document 2: Kawasaki T, Sasayama S, Yagi S, Asakawa T, Hirai T, “Non-invasive assessment of the age related changes in stiffness of major branches of the human arteries”, Cardiovasc Res, 21, 9 (1987), pp. 678-687.